Green Nanomaterials. Siddharth Patwardhan

will discharge if the charge is not continually refreshed. Hence information stored on these devices will be lost if power is lost also. The recent trend towards portable computers, smartphones, as well as portable electronics has fuelled the development of fast and compact non-volatile semiconductor data storage [42]. One of the most successful technologies in this category is flash memory, which can store data for approximately ten years without power. Flash stores data in arrays of transistors, which make up a memory cell. As with magnetic data storage, flash memory has seen massive improvements in storage densities and capacities by scaling the size of the transistors to smaller dimensions. Again, this process cannot continue indefinitely; with the physical limits being reached, new alternative technologies that utilise the quantum physical properties of nanomaterials are being explored [42].

      2.4.2.3 Nanoelectronics/spintronics

      As electronically powered technology has now become vital to our lives, we need more and more efficient methods of transferring electrical energy. Furthermore, the reduction of the size of electronic technology needs consideration. In macroscale electronics electrical energy is transferred by electrons in a metal wire moving through the lattice to form a current; charge difference can build up between two points which have a potential difference between them, which is the voltage. Electronic components have been developed throughout the 20th century, such as capacitors (collecting charge, then releasing it when a critical level is reached), diodes (semiconductors that act as a valve, allowing the electrons to flow easily in one direction but not the other), and transistors (semiconductors that are able to amplify or switch electronic flow), which have allowed our lives to be transformed with ever more complex and ingenious electronic circuity in common everyday devices. However, as size reduces, so too do the physical properties of the material. As wires get thinner, the resistance in the wire vastly increases. As we miniaturise, it may be time to consider whether the movement of the charged electron, i.e. controlling electrical devices by conveying electron charge flowing in a wire, is actually the best method in smaller devices.

      Spintronics is a new field of physics studying how the spin of electronics could be a better way to convey and control electronic signal in smaller electronic devices. Apart from the obvious benefit of miniaturisation, this has many advantages. In spintronics, there are additional characteristics of spin, and how the electron spin couples with the atomic orbital (spin–orbit coupling), how electrons can tunnel, and ultimately how these respond to an external magnetic field (figure 2.10(Ci)). All of these can be tuned, offering multiple electronic manipulation methods at the atomic level [43]. As recently as 2010 and 2013, graphite-like structured 2D semiconductor MoS₂ was shown to have photoluminescent and transistor behaviour, respectively. Building monolayered layered structures of such semiconductors, insulated and ferromagnetic materials are paving the way for complex spintronic effects that could be used in the nanoelectronics devices of the future. A 2D layered structure of MoS₂ on graphene combines the strong spin–orbit coupling of MoS₂ with the enhanced spin transport properties of graphene to form a spin field-effect switch capable of both transporting and controlling spin current (figure 2.10(Cii)). Such switches could be used to improve search engines and pattern recognition circuitry capability in the future [39]. However, spintronics is already in technology now: read heads on magnetic hard drives already use the effect of electron tunnelling from ferromagnetic layers through an insulating layer in the material. Combining the set-up of semiconductor RAM devices with magnetic domains for the data storage (instead of electronic charge) and utilising the electronic tunnelling of spintronics has led to the development of magnetoresistive random-access memory (MRAM), which has been heralded to out-compete all other forms of data storage in the future (currently not for high density, but for speed and longevity of retention) [44]. A MRAM is made up of an array of magnetic tunnel junction that holds a bit of information. Each magnetic tunnel junction is composed of a complex layered structure of two ferromagnetically orientated thin films separated by an insulating layer, with the bottom ferromagnetic film ‘pinned’ in orientation by a further antiferromagnetic thin film below.

      The key issue with the development of all the complex nanodevice technologies described above is fabrication. Producing the correct material, of the correct phase and orientation in the correct layered or nanoscale morphological structures at the nanoscale is very problematic. Adding to that the ever-increasing need to fabricate such intricate complex materials and systems in a reproducible and sustainable way, for them to be industrially viable for applications, presents a formidable challenge.

2.4.3 Consumer products

      A consumer product inventory has listed in total 1829 products containing nanomaterials, using in total 47 types of nanomaterials (www.nanotechproject.org/cpi) [45]. These products are produced by over 600 companies, which are spread over more than 30 countries globally. Of the nanomaterials used, titania, silica, zinc oxide and silver are the most common both in terms of total mass, as well as the number of products they are found in. When considered on mass basis, titania, zinc oxide and silica are the top three most manufactured nanomaterials.

      2.4.3.1 Titania and zinc oxide

      Titania and zinc oxide form the active components of sunscreens due to their ability to specifically interact with UV light (absorb or reflect) but not visible light. As discussed in the optical properties section above, it is their size and specific chemistry that enables them to block UV rays (both UV-A and UV-B in the range of 320–400 nm and 290–320 nm, respectively). The interactions of these two materials arise from their band gaps, which allow them to absorb UV rays; the specific wavelengths of absorption depend on the actual band gap (which in turn depends on the chemistry and sizes) of the materials. Generally, titania is most effective in blocking the UV-B range, while ZnO blocks the UV-A range. Using nano-sized (<100 nm) titania or ZnO makes them transparent in visible light, as well as enabling control over their band gaps (which become dependent on the particle sizes, as seen in section 2.2).

      2.4.3.2 Silica

      Silica is another widely used nanomaterial in applications such as toothpastes, tyres and cosmetics. One of the key properties in its success is its inertness under most conditions of use. Silica is resistant under acidic conditions and most organic reagents—it can only be degraded by strong caustic solutions (pH > 10) or hydrofluoric acid. As silica can withstand high temperatures similar to glass, it provides excellent improvement in thermal properties when blended with polymers. This, coupled with its added mechanical strength, means silica is an ideal component in tyre and composite applications. Further, silica (in most forms) has been ‘generally regarded as safe (GRAS)’ by regulating bodies in the USA and Europe, and hence is widely used in food and cosmetics. In those applications, silica improves the rheological (flow) properties of food and cosmetic products. Finally, its specific porous structure and the ability to absorb water/moisture makes it highly useful in desiccant applications where even a small amount of moisture needs to be avoided (e.g. electronics packaging and table salt).

      2.4.3.3 Silver nanoparticles

      Silver nanoparticles are another major nanomaterial used in consumer products, mainly for antimicrobial protection. The use of particulate silver in healthcare for the treatment of wounds has been recorded as far back in time as Ancient Greece. Silver is a well known antimicrobial agent, being used in antibacterial, anti-fungal and antiseptic creams, dressings and medical equipment coatings. While silver is an inert metal to us, it is highly toxic to microbes by damaging enzymes in pathogen membranes. The advantage of nanoparticulate silver is that the increased surface area increases the relative activity. It is now common to find silver nanoparticles in simple cheap wound dressings such as sticking plasters. The antimicrobial effect of silver nanoparticles is used in a wide range of consumer